Generally speaking you should go more for a scale out scenario (higher number of VMs with lower CPU configuration) than vice versa in my experience. This is because IRQ handling in XenServer on a guest VM level is restricted to only 1 CPU. Hence if there is a log of IRQs a scaled-up VM could come to its limits. Max out hardware by higher number of VMs with lower number of vCPUs each should get better values.

Whatever the hypervisor XenServer, ESX 4/5, Hyper-V R2SP1, no reports of showing more than 110/120 VMs per hypervisor.

ProjectVRC – www.projectvrc.com has a number of hypervisor scalability reports created using LoginVSI the industry standard to measure user performance and the maximum amount of sessions a host can support with a decent performance.

Enable NUMA on the hypervisor and add the below options to the /boot/extlinux.confboot file to pin the Dom0 vCPUs to a single NUMA node. (numa=on dom0_vcpus_pin dom0_max_vcpus=4) (Stay with default 4 vCPUs and use the 2940M for Dom0 memory)

In order to compile the SPA reports just change to the SPA install dir of each machine (exchange and XA VMs) and execute: spacmd compile “System Overview” afterwards you can analyze the report within SPA by navigating to “reports”->”current”.

XenTop – Displays real-time information about a Xen system and domains

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